Cracking, warping, and surface deterioration often occur during firing of ceramic tapes. In addition, for tapes containing volatile components out-diffusion of materials can be a significant problem. A variety of approaches have been applied in attempts to solve these problems. However, no solution has been completely satisfactory. In a conventional approach a flat refractory cover plate (or setter) is placed on top of the unfired (xe2x80x9cgreenxe2x80x9d) ceramic tapes in order to prevent severe warping or wrinkling of the tapes during bum-out of organic binders in the tapes and sintering of the tapes at high temperatures (M. Asakura et al. (1982) Annual Report of the Engineering Research Institute Faculty of Engineering, University of Tokyo, Vol. 41, pp. 1 85-190). The cover plates used must be heavy enough to press the green ceramic tape flat during the sintering process, but the weight of the cover plate generates shear forces on the faces of the relatively fragile tape as it contracts (Kaga et al. U.S. Pat. No. 5,620,637 and Sawada et al. U.S. Pat. No. 5,527,501). The forces imposed on the tapes by the use of the cover plate makes them very susceptible to fracturing and destruction, particularly if the composition contains a volatile or liquid phase, such as PbO. In addition, the cover plates, if not completely chemically inert and compatible with the tapes at high temperatures, can bond or stick to the ceramic tapes during firing.
Many commercially important ceramic tapes have a volatile phase, such as PbO in piezoceramic-based tapes, e.g., PZT or PLZT ceramic tapes. At high temperatures, PbO will readily diffuse into the cover plate from these tape, resulting in a lead-deficient PZT or PLZT tape with very poor functional properties after firing. Loss of PbO from these materials adversely affects electromechanical coupling factors and dielectric constants (Webster A. H. et al. (1967) J. Am. Ceram. Soc. 50(9):490).
One way to prevent or decrease cracking and sticking is by coating the green ceramic tape with a suitable dry refractory ceramic powder (setter powder) by sprinkling, sifting, rubbing and/or brushing the ceramic powder onto and between green tapes or other shaped green ceramic articles (e.g., Fries, R. and Moulson, A. J. (1994) J. Mater. Sci.: Mater. in Electronics 5:238). One major problem with this method is non-uniformities in powder dispersion which lead to surface deformations and defects in the thin ceramic plates that cannot be tolerated in many applications. It is not practical to disperse powders uniformly over the delicate, thin, unfired tape and keep the powders uniformly dispersed as the tape is covered with the cover plate, transported and then fired. The use of loose powders in tape processing is additionally disadvantageous because it is inherently a slow and tedious process and can be wasteful with respect to the powders employed. Further, the use of loose powders, which can become airborne, in a manufacturing process can represent a respiratory hazard.
A second approach to improving firing of ceramic tapes or other shaped articles is the use of a solvent to disperse a thin layer of setter powder on tapes by dipping or wet spraying the tapes prior to firing. This method may be used to achieve acceptable uniformity. The solvent used must, however, be carefully selected. The solvent can exhibit poor wetting characteristics that result in particle agglomeration or it can dissolve binders and additives in the green tape destroying the integrity of the tape.
Other methods for protecting tapes rely on application of a thin ceramic coating to the cover plate, usually in the form of a relatively inert layer of zirconia or other refractory ceramic or as a thin platinum coating or foil (Hind, D. and Knott, P. R. (1997) in Electroceramics: Production, Properties and Microstructures, W. E. Lee and A. Bell (Eds.), The Institute of Materials, London, UK, p. 107; and Stevenson, J. W. et al. (1994) J. Am. Ceram. Soc. 77(9):2481). Platinum foils, refractory ceramic plates or setter powder can also be placed between stacked ceramic tapes during firing. These methods tend to have high materials and manufacturing costs. Some manufacturers supply cover plates with textured surfaces to provide only xe2x80x9cpoint contactsxe2x80x9d with the green tapes. These methods attempt to eliminate the need for uniformly disperse powder, prevent out-diffusion and unwanted bonding of tapes to the cover plate. A major limitation of these methods, however, is that they are not effective for use with large area ceramic tapes with thicknesses of a hundred microns or less that are required in many important current applications. The inherent shear forces and increased friction produced by cover plates having permanent barrier coatings or textured surfaces are still too high for more delicate thin ceramic tapes and their use results in cracked and broken tapes.
In a related method, Busse et al. in U. S. Pat. No. 5,359,760 report the use of solid ceramic separator plates (setters) with high PbO content to prevent out-diffusion of volatile PbO from PLZT and PZT ceramic tapes. These plates or setters lose lead and must be replaced relatively frequently and cannot maintain their flatness. Further, these solid separator cover plates are not expected to be useful for large area very thin tapes because the friction and shear forces remain too high.
As noted above, loss of PbO from PLZT and PZT tapes is a significant problem during sintering. A common way of reducing PbO loss is to place the PLZT or PZT green tapes in a closed crucible surrounded with lead-containing atmosphere powders (Snow, G. S., (1973) J. Am. Ceram. Soc. 56(2):91; Snow, G. S., (1973) J. Am. Ceram. Soc. 56(9):479; Kingon, A. I. and Clark, J. B. (1983) J. Am. Cerm. Soc. 66(4):253). Other approaches have focused on decreasing sintering temperature by adding liquid phase agents (Wittmer, D. E. and Buchanan, R. C. (1981) J. Am. Cerm. Soc. 64(8):485; Cheng et al. (1986) J. Mater. Sci. 21:571; Zhilun et al. (1989) J. Am. Cerm. Soc. 72(3):486). The presence of a liquid phase promotes densification kinetics during sintering (James, A. D. and Messer, P. F. (1978) Trans. J. Br. Ceram. Soc. 77(5):152; and German, R. M. (1985) Liquid Phase Sintering, Plenum Press, New York, N.Y.). Excess PbO can act as a liquid phase agent and has been reported to affect grain size and density of piezoceramics (James, A. D. and Messer, P. F. (1978) Trans. J. Br. Ceram. Soc. 77(5):152; Chian, S.-S. et al. (1981) Am. Ceram. Soc. Bull. 60(4):484). It has also been reported that high amounts of excess PbO can only increase densification rates in the early stage, and will lower the final density of ceramic bodies due to evaporation of PbO at elevated temperatures (Kingon, A. I. and Clark, J. B. (1983) J. Am. Ceram. Soc. 66(4):256).
PLZT and PZT tapes are often co-fired in multiple layer stacks. In this case, the tapes may stick together and be difficult to separate without fracturing. This problem can be solved by placing platinum foils, solid PZT plates, or setter powder between tapes and between the tapes and the cover plates. The use of platinum foil and PZT plates is not economical for mass production of tapes and the disadvantages of the use of loose setter powder has been discussed above.
The present invention provides an improved method and low cost buffer sheets useful in the method for firing ceramic tapes or other shaped articles, particularly thin tapes and articles with thin walls, and particularly those combining large surface area with thin and more particularly for PLZT and PZT tapes.
The present invention provides an inexpensive, non-toxic, simple and reliable method for obtaining uniform and thin layers of particles or powders that are particularly useful an anti-sticking and anti-friction coatings during the fusion and densification (or sintering) of ceramic tapes and other shaped articles. The method is particularly useful for sintering of large area, very thin ceramic tapes. In application to very thin tapes and large surface area thin materials, the present invention provides a satisfactory firing method where other approaches have failed. The method facilitates application of a thin and uniform layer of non-sinterable (i.e., refractory) particles on the green tapes or articles to provide a reduction in shear stress from cover plates (i.e., refractory) particles on the green ceramic articles to provide a reduction in shear stress from cover plates. The method provides sufficient protection to allow the production of relatively large area thin ceramic plates not previously attainable by any comparable method. The method is also particularly useful for ceramic articles that contain volatile components or that require sintering in the presence of a liquid phase. In a specific embodiment, the method of the present invention is useful for the sintering of piezoceramic tapes and multi-layer stacks of these ceramic tapes. The methods of this invention are particularly useful in the manufacture of crack-free ceramic tapes with thicknesses in the range of 100""s of microns or less with minimal surface defects.
In a preferred embodiment, the method employs setter powder deposition sheets which are thin sheets having a binder layer which contains refractory powders or particles (e.g., setter powder). The setter powder deposition sheets are placed between unfired (or green) ceramic shaped articles, including green tapes and green plates, and top and bottom refractory ceramic cover plates or placed between stacked layers of green ceramic articles, particularly between green tapes. Use of the sheets during firing deposits a uniform layer of refractory particles or powder on the surface of the green article minimizing or preventing fracturing. The sheets are particularly useful in sintering of ceramic tapes when ceramic cover plates are used during burning-out and sintering to keep tapes flat. The sheets also minimize or prevent sticking of the ceramic articles to be fired to each other or to the cover plates and minimizes surface defects therein. Further, the buffer sheets when used with cover plates minimize or prevent loss of volatile components, from ceramic articles that are fired such as PbO from piezoelectric ceramic (e.g., PZT or PLZT ceramics), during burn-out and sintering.
Setter powder deposition sheets can be economically fabricated in large quantities using conventional methods, e.g., tape casting, processed using conventional paper handling techniques and conveniently stored in pre-cut sheets. These sheets are sintering aides that provide a thin uniform layer of setter powder on green ceramic tapes during firing. During burn-out of the thermally fugitive components of the buffer sheets, free ceramic particles or powders are left on the surface of the ceramic shaped article in a very uniform thin layer. These refractory powders or particles are selected so that they have a tendency to resist bonding or fusion and thereby provide a slippery medium between the ceramic article and the cover plates (or other ceramic tapes or articles) during firing. The refractory particles or powders originating from the setter powder deposition sheets and remaining after firing can be easily washed or blown off the sintered tapes or other articles. The sheets are easy to store and dispense and can greatly reduce problems with clean-up and respiratory hazards for airborne particles.
The method of this invention also avoids pile-up of particles of setter powder during shrinkage of the green tape on firing. Particle pile-up can results in warping and surface defects in the sintered tapes or other shaped articles. The binder used in the setter powder deposition sheets is selected to have a lower burn-out temperature than any binder employed in the green ceramic tapes or other shaped articles. The binder used in the setter powder deposition sheets preferably has a burn-out temperature that is significantly lower than that of any binder used in the green ceramics. The binder used in the setter deposition sheets is preferably selected so that the green strength of the green ceramic article to be sintered remains high during binder burn-out of the buffer sheets. It is believed that the setter powders tend to form a weak bond with the green ceramic tape and that the separation between individual setter particles is reduced as the green tape contracts.
In a specific embodiment, setter powder deposition sheets are cast by conventional tape casting methods on a suitably carrier, e.g., Mylar (Trademark, DuPont) foil. The slurry used for tape casting comprises setter powder and a binder in an aqueous or non-aqueous carrier. Preferred slurries also contain plasticizers and/or wetting agents. Slurries may also contain inert fillers. Preferred carriers are inert with respect to the various slurry components and are appropriate for use with conventional methods for preparing ceramic tapes or sheets, including tape casting techniques. A more preferred support material for forming sheets is Mylar foil. In preferred embodiments, support material is provided with a thin coating of a wetting agent to facilitate deposition of the slurry.
The method of the present invention is not specifically intended to prevent shrinkage of green ceramic tape or other shaped articles.
This invention includes setter powder deposition sheets, methods of making these sheets and methods of using these sheets in ceramics processing.